Extreme Phosphorus Scarcity and its Grip on Ancient Life

Life’s list of essential nutrients is long, but carbon, nitrogen, and phosphorus are the big three. Carbon and nitrogen, both easily extracted from the atmosphere, have usually been in ample supply in the ocean over Earth’s history. Carbon dioxide readily dissolves in seawater, and that carbon is then converted to the molecules of life through photosynthesis. Nitrogen is nearly 80 percent of the air we breathe, and diverse microorganisms are able to convert nitrogen to compounds more widely useful to life.

Phosphorus is much harder to get: it must be delivered to the oceans by rivers fed through the weathering of rocks exposed on the continents. To keep life thriving for billions of years, the planet’s initial allotment of phosphorus must be recycled time and again. These repeated shots of phosphorus into life’s collective arm require one of Earth’s most dramatic displays — the episodic formation and subsequent erosion of massive mountain chains when continents collide.

Because so much tectonic oomph is required to replenish phosphorus, scientists have long assumed that the history of life on Earth is written partly by factors that control how fast this nutrient is moved from land to sea. But the not-so-little secret about the vital role of phosphorus is that its long-term availability in the ancient ocean has never been well documented.

Now, a study published in Nature reveals the first direct evidence for extreme and protracted scarcity of phosphorus in the ancient ocean. Former UC Riverside graduate student Chris Reinhard, now an assistant professor at Georgia Institute of Technology and lead author of the study, and Tim Lyons, a UC Riverside distinguished professor and leader of the Alternative Earths Team of the NASA Astrobiology Institute, worked with an international group of scientists to assemble an unprecedented set of phosphorus data generated by chemical analysis of roughly 8,000 shale samples spanning the world — and more than three billion years of history.

These data show phosphorus limitation in the surface ocean over most of that interval, likely related to the oxygen-poor conditions in the deep sea. That extreme scarcity may help explain why simple microscopic life, such as bacteria, dominated these waters — and why complex life did not arise until much later.

“The trend in our data is quite remarkable,” Lyons said. “It suggests that phosphorus was lost to the sediments of the deep ocean through processes we don’t fully understand, denying life in the shallow waters easy access to this essential nutrient.”

The study further suggests that phosphorus in shallow waters increased around 800 million years ago, coincident with a significant spike in oxygen content of the oceans and atmosphere. This timing suggests phosphorus scarcity may also explain the long-delayed oxygenation of Earth’s surface. Greater phosphorus availability in the oceans supports higher levels of primary production and photosynthesis in the surface ocean, with the ultimate effect of releasing more oxygen to the atmosphere. At the same time, complex life such as algae exploded in the oceans for the first time—followed not long after by the rise of animals.

“We are now in a wonderful position to unravel the captivating chicken-and-egg relationships among the evolution of life, the rise of oxygen, the shifting availability of phosphorus in the oceans, and even the possibility of episodic nitrogen limitation,” Lyons said. “My money is on the important role of plate tectonics and, 800 million years ago, the breakup of a supercontinent. The next exciting step is to extrapolate these new views to help guide NASA’s search for life on exoplanets light-years beyond our solar system.”